In order that low loss optical fibers which are used in communications systems are not diminished in their effectiveness, the fibers must be connected through intermateable connectors which preserve those low losses. For optical fiber ribbons, connectors comprise grooved chips which hold a plurality of fibers of one ribbon in alignment with fibers of another ribbon. Such a connector is shown for example in U.S. Pat. No. 3,864,018 which issued on Feb. 4, 1975 in the name of C. M. Miller.
For single fiber cables, connections may be made through a connector which is referred to as a biconic connector. See, for example, an article entitled "Interconnection for Lightguide Fibers" which was authored by T. L. Williford et al. and which appeared in the Winter 1980 issue of the Western Electric Engineer beginning at page 87. That connector includes a coupling having a housing in which is mounted a biconic alignment sleeve. The sleeve includes two truncated, conically shaped cavities which communicate with each other through a common plane which has the least diameter of each cavity. Each of two optical fibers to be connected is terminated with a plug assembly which includes a plug comprising a primary pedestal or truncated, conically shaped end portion which is adapted to be received in one of the cavities of the sleeve. Each fiber extends through the plug in which it is mounted and has an end which terminates in a secondary pedestal of the plug. The plug is held in a cap having an externally threaded portion adapted to be turned into an internally threaded entrance portion of the housing. At least portions of the conically shaped surfaces of the plug and of the sleeve serve as alignment surfaces and are intended to be conformable. The truncated, conically shaped portion of the plug is urged by a spring which is disposed about the plug into seated engagement with the wall defining the cavity in which it is received while the cap is turned into the housing. Minimal loss between the connected fibers is achieved when the optical fibers which are terminated by the plugs are aligned coaxially and when the fiber end faces, each of which is planar, contact in a common plane.
One problem with these kinds of connectors relates to the mounting of the plugs within the sleeve. In some installations, it becomes very difficult, if not impossible, to hold the cable while turning the cap into the housing of the coupling. If the plug is not held while the cap is turned into the housing in which the sleeve is disposed, the plug will turn with the cap. If the plug turns, the end face of the plug and hence of the optical fiber terminated therewith may abrade against the plug and optical fiber already in the coupling, possibly causing damage to the optical fiber. This problem has been overcome by providing each plug assembly with a key which is mounted on the plug and which projects toward the free end of the plug in which the end of the optical fiber terminates. The sleeve has each one of its ends provided with a notch with the key of each plug assembly being adapted to be disposed in a notch at one end of the sleeve to prevent rotation of the plug with respect to the sleeve as relative rotational motion is caused between the cap of the plug assembly and the coupling.
Another problem relates to the potential for disconnection of the optical fiber end faces and the disengagement of the conically shaped plug end portions from the sleeve. It will be recalled that the conically shaped portion of the plug is biased inwardly into the housing by a spring. Should sufficient tensile force be applied inadvertently axially to the cable which is terminated by the plug, the plug will be moved in a direction outwardly from the center of the sleeve causing effectively an optical disconnection of the optical fiber end faces and hence a disconnection of optical transmission.
Also, because of the construction of the housing, the plug, upon the application of forces to the cable in a direction transversely of the longitudinal axis of the connector, will cause the plug to tilt about a fulcrum disposed between the center of the sleeve and the end of the cap. This results in a canting of the end face of the plug and angular spacing thereof from the other plug end face thereby causing optical disconnection across the plug ends.
In attempting to provide a solution to the problem of unintended longitudinal and turning movement of the plug, one must be mindful of the problem of compatibility. With many biconic connectors already in use, it would be imprudent to provide plug assemblies which overcome the problem of such unwanted movement but which are not compatible with plugs and sleeves already in use. Another kind of optical fiber connector in which unwanted optical disconnection is prevented is disclosed and claimed in commonly assigned application Ser. No. 068,585 filed of even date herewith in the names of A. W. Carlisle, et al.
Seemingly, the prior art has not yet offered a simple solution to the problem of preventing inadvertent movement of conically shaped plugs which terminate optical fibers in a biconic coupling in order to avoid unwanted optical disconnection. The sought-after connector should be one which prevents optical decoupling upon the application of both axial and transverse forces. Desirably, the plug assembly and the sleeve of the sought-after connector are ones which are compatible with connectors that already are in use in the field.